You should be able to get a cable that has a 3.5mm jack at one end, and the red & white audio connectors at the other. Plug the 3.5mm end into the sound output jack on your computer, and the red & white connectors into the corresponding connectors on your stereo. Put the stereo into auxiliary mode and you're done.
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The simplest way to connect a standard computer to a home theatre system is to use a stereo mini-jack (male) to dual RCA (male) cable. Connect the mini-jack to your computer's "line-out" headphone jack and the RCA jacks to your home theatre.
NOTE: Make sure that the volume is turned ALL THE WAY DOWN on your home theatre system BEFORE you connect the jack.
This setup won't give surround sound, but unless you have an HDMI output, optical output or spdif output, then 2 channel stereo is all you're going to get. Don't worry though, it'll be plenty loud!